Not sure if you've solved your problems yet, but I recently went through this with my Xbox One after upgrading to a new Xfinity Gateway. I was getting dropped from games and group chat constantly. Basically, it was unplayable.
I tried all the fixes mentioned above, and it never worked. But, once I put my Xfinity into "Bridge" mode, and then hooked up my wireless router to it, everything has been fixed. Bridge mode basically turns off the wireless capabilities of the Xfinity Gateway and makes it just a modem.
It's like night and day. I have a really nice ASUS wireless router that has all the settings you need to make the Xbox One work perfectly.
Best of luck.
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